Deposition of cadmium by chemical reduction



A 2,790,733 Patented Apr. 30, 1957 DEPOSITION OF CADMIUM BY CHEMICAL REDUCTION Dwight E. Couch, Washington, D. C., assignor to the United States of America as represented by the Secretary of the Navy N Drawing. Application December 14, 1955, Serial No. 553,163

8 Claims. (Cl. 117-430) (Granted under Title 35, U. S. Code (1952), see. 266) Other objects and many of the attendant advantages of this invention will be readily appreciated as the same become better understood by reference to the following detailed description.

According to the invention, deposits of cadmium can be obtained on iron, copper, brass, and copper or brass plated metals, without the use of an outside source of electric current, by means of treatment in a fused-salt solution. Because of the low melting point of cadmium metal, 321 C. (610 F.), there are a number of fusedsa-lt baths that can be used in reactions of this type.

One reducing agent that Will reduce the cadmium ion from its compounds is fused sodium formate. Cadmium sulfate, chloride, and iodide are all reduced rapidly by fused sodium formate at 270 C. (518 F.), giving a gray powder which collects as a sponge when heated for a short while. Cadmium prepared by reduction using fused sodium formate analyzes 97 percent pure cadmium.

The deposition of cadmium coatings on articles of iron, copper or brass can be obtained with the baths of the present invention. All of these metals can be coated from the same bath and are coated by dipping the cleaned metal pieces into the baths described. Typical baths which were found to be operative are as follows:

most of thesereactions are used only in analytical chemistry, or not at all. However, there are two methods of chemical reduction which have been used commercially: the reduction of silver employed in silvering mirrors and the reduction of nickel by hypophosphite in hot solutions, commonly known as the Electroless process. By the proper regulation of conditions, this process can be made to reduce nickel only on specially catalyzed, or active surfaces.

Deposition by chemical reduction has certain advantages over the electrodeposition of metals. Because chemical deposition proceeds in recessed areas and deep holes about as fast as it does on exterior surfaces, the deposit does not build up on sharp edges or form trees, as it does in the electrolytic method. The rate of deposition by chemical reduction is slower than by electrodeposition; consequently, these deposits are most useful where thin deposits or coatings will sufiice. Deposition of cadmium by chemical reduction has many potential applications, one of which is to provide corrosion protection to small metal parts with irregular or complex shapes that cannot be plated satisfactorily by conventional means.

It is an object of the present invention to provide a process of depositing adherent coatings of cadmium upon other metals, without the application of any source of outside electric current, from fused-salt solutions containing cadmium salts;

It is another object of the invention to provide a method of reducing cadmium ions to the free metal by reduction is fused-salt solutions.

A further object of the invention is to provide corrosion protection to metal parts having irregular or complex shapes by depositing a uniform coating of cadmium thereon by chemical reduction.

potassium formate, operating temperature range from About .l'% to 11% of cadmium in the form of its oxide or salts, about 12% to 70% acetamide, about 19, to 88% sodium formate, operating temperature range from 200- 220 0;

About .1 to 11% cadmium oxide or salt, about 89 to 99.9% potassium formate, operating temperature 200- 280 C.;

About .1 to 12% cadmium oxide or salt, about 88 to 99.9% sodium formate, operating temperature 270- 300 0.;

About .1% cadmium salt, about 11% sodium oxalate, about 88.9% potassium formate, operating temperature 285-300 C.

Other operative baths may be prepared using sodium formate and potassium formate in the same solution, or baths may contain acetamide, potassium formate and sodium formate.

The fused baths set forth produced uniform coatings of cadmium on irregular shaped pieces of iron, copper and brass, making them corrosion resistant.

These baths have also been used for the production of finely divided or powdered cadmium. If cadmium powder is desired the percentage concentration of cadmium salt and the operating temperature should be at the maximum amounts set forth above. After the reduction has gone to completion the bath is diluted by pouring it into an excess of water. Then the cadmium metal is filtered, washed and dried.

If cadmium formate is used as the source of cadmium the anion build-up in the fused bath, such as results when cadmium chloride is used, can be eliminated.

The invention set forth herein represents a new method of obtaining cadmium coatings on iron, copper, brass, and the like without the use of electric current, and also gives a method whereby cadmium can be reduced to the free metal powder from cadmium salts.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

l. A process for producing uniform adherent coatings of cadmium on irregular shaped surfaces of metals selected from the group consisting of iron, copper and brass, without the application of an outside source of electric current, which comprises immersing the metal articles to be coated in a fused salt bath maintained at a temperature from about 200 C. to 300 C. containing from about 88 to 99.9 percent by weight an alkali metal formate and from about .1 to 12 percent by weight of a cadmium compound which ionizes, for a sufiicient time to form a desired coating.

2. A process for providing uniform adherent coatings of cadmium upon irregular shaped surfaces of metal selected from the group consisting of iron, copper and brass, without the application of an outside source of electric current, which comprises immersing the metal articles to be coated in a fused salt bath maintained at a temperature from about 200 C. to 300 C. containing from about 88 to 99.9 percent by weight of an alkali metal formate and from about .1 to 12 percent by weight of a cadmium compound selected from the class consisting of the chloride, oxide, iodide, acetate, carbonate and sulfate of cadmium, for a sufficient time to form the desired coating.

3. The process of claim 2 in which said fused salt bath contains acetamide.

4. A process for producing uniform adherent coatings of cadmium on irregular shaped surfaces of metals selected from the group consisting of iron, copper and brass, without the application of an outside source of electric current, which comprises immersing the metal articles to be coated in a fused salt bath maintained at a temperature from about 170 C. to 300 C. containing an alkali metal formate in an amount from about 19 to 90 percent by weight, a cadmium compound which ionizes in an amount from about .1 to 11 percent by weight and acetamide in an amount from about 10 to percent by weight, for a sufficient time to form a desired coating.

5. A fused salt bath for the production of substantially pure metallic cadmium by the reduction of cadmium ions to free cadmium metal in a fused-salt solution, said bath comprising from about 88 to 99.9 percent by weight of an alkali metal formate and from about .1 to 12 percent by weight of a cadmium compound which ionizes maintained at a temperature from about 200 C. to 300 C. until the reduction has gone to completion.

6. A fused-salt bath for the production of substantially pure metallic cadmium by the reduction of cadmium ions to free cadmium metal in a fused-salt solution, said bath comprising from about 19 to 90 percent by weight of an alkali metal formate, from about .1 to 11 percent by weight of a cadmium compound which ionizes and from about 10 to 70 percent by weight of acetamide maintained at a temperature from about C. to 300 C. until the reduction has gone to completion.

7. A process for producing substantially pure metallic cadmium in finely divided form by preparing a fused-salt solution containing from about 88 to 99.9 percent by weight of an alkali metal formate and from about .1 to 12 percent by weight of a cadmium compound which ionizes, said solution being maintained at a temperature from about 200 C. to 300 C. until the reduction of cadmium ions to free cadmium metal has gone to completion, diluting said fused-salt solution after the reduction has gone to completion by pouring it into an excess of water, and filtering, washing and drying the cadmium metal from said diluted fused-salt solution.

8. A process for producing substantially pure metallic cadmium in finely divided form by preparing a fused salt solution containing from about 19 to 90 percent by weight of an alkali metal formate, from about .1 to 11 percent by weight of a cadmium compound which ionizes and from about 10 to 70 percent by weight of acetamide, said solution being maintained at a temperature from about 170 C. to 300 C. until the reduction of cadmium ions to free cadmium metal has gone to completion, diluting said fused-salt solution after the reduction has gone to completion by pouring it into an excess of water, and filtering, washing and drying the cadmium metal from said diluted fused-salt solution.

References Cited in the file of this patent Hoge: Metal Progress, vol. 52, November 1947, pp. 81943. l 

1. A PROCESS FOR PRODUCING UNIFORMADHERENT COATINGS OF CADMIUM ON IRREGULAR SHAPED SURFACES OF METALS SELECTED FROM THE GROUP CONSISTING OF IRON, COPPER AND BRASS, WITHOUT THE APPLICATION OF AN OUTSIDE SOURCE OF ELECTRIC CURRENT, WHICH COMPRISES IMMERSING THE METAL ARTICLES TO BE COATED IN A FUSED SALT BATH MAINTAINED AT A TEMPERATURE FROM ABOUT 200*C. TO 300*C. CONTAINING FROM ABOUT 88 TO 99.9 PERCENT BY WEIGHT AN ALKALI METAL FORMATE AND FROM ABOUT .1 TO 12 PERCENT BY WEIGHT OF A CADMIUM COMPOUND WHICH IONIZES, FOR A SUFFICIENT TIME TO FORM A DESIRED COATING. 